The circuit of my invention may be used to control and maintain any medium between desired limits. It could be used to control temperatures, flow rates, light levels, the position of garage doors, or any of a number of other functions where upper and lower limits can be detected with a suitable probe or switch. In this application the control circuit is incorporated into a preferred embodiment wherein the liquid level in a sump is maintained by means of upper and lower limit probes. The invention is not limited to sump pumps, however, and may be incorporated into a wide variety of different devices.
In the prior art relative to liquid level control several approaches have been known including mechanical float switches, capacitive switches, sonic switches, and conductivity type switches. In the preferred embodiment conductivity type switches are utilized because they allow the switching device itself to be mounted remotely from the liquid thus reducing corrosion problems. Such switches also permit the control circuit to be sealed against weather and other ambient conditions. In the past, conductivity type switches have often encountered problems with electrolysis of the probes caused by the flow of current therethrough. For a reverse flow there has often been a problem with plating of the probes with undesirable substances. My invention overcomes these problems with the circuit described hereinafter.